CN100520672C - DDS signal source amplitude-frequency characteristic compensation method and related DDS signal source - Google Patents
DDS signal source amplitude-frequency characteristic compensation method and related DDS signal source Download PDFInfo
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- CN100520672C CN100520672C CNB2007101752867A CN200710175286A CN100520672C CN 100520672 C CN100520672 C CN 100520672C CN B2007101752867 A CNB2007101752867 A CN B2007101752867A CN 200710175286 A CN200710175286 A CN 200710175286A CN 100520672 C CN100520672 C CN 100520672C
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Abstract
This invention offers a kind of amplitude frequency compensation method for the DDS signal source through the digital filter and the corresponding DDS signal source and adds a digital filter module and a selecting switch between the present DDS signal source's RAM search table and the digit and analog converter. Before the DDS signal source is used in reality set up a selecting switch and make the RAM finding table found the output data and then send the DAC directly and do the digital and analog convert. Then it can get the amplitude frequency characteristic function of signal source by testing the output signal and get the reciprocal of this function and then calculate it by using the frequency sampling method and then get the tapping coefficient of digital filter module. Write the tapping coefficient in the tapping coefficient finding table of digital filter module and set up the selecting switch again then let the output data of RAM finding table output to the DAC through the digital filter module and then do the digit and analog converter so it can realize the amplitude frequency characteristic correction for the DDS signal source.
Description
Technical field
The present invention relates to DDS signal generator field, particularly a kind of can effectively the improvement because of ripple in DAC retention performance and the elliptic filter passband rise and fall uneven method of caused signal amplitude versus frequency characte and device thereof.
Background technology
DDS (Direct Digital is synthetic) is the third generation frequency synthesis technique after direct frequency synthesis technique and phase-locked ring type frequency synthesis technique, it relies on that its relative bandwidth is wide, switching time is short, resolution is high, output phase is continuous, can produce wideband orthogonal signal and other multiple modulation signal, able to programme, totally digitilized and control characteristics such as flexible, be widely used in the signal generator field in recent years.DDS signal source basic structure mainly is made up of phase accumulator, waveform RAM, digital to analog converter (DAC) and low-pass filter etc.The phase increment addition that phase accumulator is determined with frequency control word under sampling clock control; The high position output of phase accumulator realizes the conversion of waveform phase to amplitude as the address of waveform RAM; Wave data converts analog quantity to through DAC, by the level and smooth relatively waveform of low-pass filter output.
Low-pass filter plays a part remain valid component, clutter reduction in DDS, the quality of output waveform quality is played crucial effects.For sinusoidal signal, according to the nyquist sampling theory, the harmonic component of output signal will appear at kf
s± f
0Place, wherein f
sBe the frequency of sampling clock, f
0It is output signal frequency.In the DDS signal source, often select for use the precipitous elliptic filter of transient characteristic to finish the filtering of offset of sinusoidal signal, guarantee that the harmonic wave of sinusoidal signal suppresses, rise and fall but the amplitude versus frequency characte of elliptic filter all has ripple in passband and stopband.In addition, be not to be desirable unit impact sampling because at present DAC realizes, but adopt zeroth order to keep technology, its transport function has the envelope trait of sinx/x.Therefore, based on its passband amplitude versus frequency characte unevenness of signal source of DDS.
At present, the DDS signal source often adopts the LC resonant circuit and carries out the correction of amplitude versus frequency characte in conjunction with the method for software, but because inductance, the deviation of capacitance values and the influence of circuit board stray capacitance and stray inductance, LC resonant circuit and the calculated value realized have certain deviation, and need the professional designer to regulate at every turn, waste time and energy.And the method that software is proofreaied and correct can only realize the correction on the frequency, can't realize the amplitude correction of waveforms such as frequency sweep, frequency modulation.
Summary of the invention
The objective of the invention is provides a kind of technology of carrying out the amplitude versus frequency characte compensation by digital filtering for DDS signal generator (claiming the DDS signal source again).
The present invention sets up a digital filtering module and a selector switch (referring to Fig. 1) between the RAM look-up table of existing DDS signal source and digital to analog converter (DAC), the amplitude information of RAM look-up table output is switched by selector switch, selection is that amplitude information is carried out carrying out digital-to-analog conversion again after the amplitude precorrection through digital filtering module, still directly amplitude information is delivered to DAC and is carried out digital-to-analog conversion.The DDS signal source is carried out amplitude versus frequency characte compensation (promptly proofreading and correct) by digital filtering before dropping into practical application, concrete aligning step is as follows:
1) selector switch is set, selects that the data of RAM look-up table output are directly delivered to DAC and carry out digital-to-analog conversion;
2) obtain the amplitude versus frequency characte function of this signal source by the output signal of test DDS signal source;
3) the amplitude versus frequency characte function that is obtained is got inverse;
4) inverse according to the amplitude versus frequency characte function utilizes frequency sampling method to calculate the tap coefficient of digital filtering module;
5) tap coefficient is inserted the tap coefficient look-up table of digital filtering module;
6) selector switch is set, outputs to DAC after allowing the data of RAM look-up table output by digital filtering module.
Another object of the present invention provides a kind of above-mentioned DDS signal source of carrying out amplitude-frequency characteristic compensation method that realizes.DDS signal source of the present invention mainly is made up of Wave data synthesis module, digital filtering module, selector switch, DAC and signal condition module, several parts of central processing module (CPU) and corresponding with it process software, and division is as follows:
1) the Wave data synthesis module is mainly by phase accumulator, RAM look-up table, sampling clock and be aided with the logical controlling circuit and form, and the fast wave graphic data of being responsible for finishing required frequency under the synchronous situation of sampled clock signal produces function.
2) digital filtering module is a core of the present invention, as Fig. 2, it comprises data storage cell, tap coefficient look-up table, multiplier, totalizer, state control logic unit: the sampling clock in the Wave data synthesis module is connected with totalizer with the state control logic unit respectively; Output control signal in state control logic unit is controlled data storage unit, tap coefficient look-up table and totalizer; The output of the output of data storage cell and tap coefficient look-up table is delivered to multiplier and is finished multiplication operations, and totalizer is delivered in its output again, by the amplitude information behind the totalizer output calibration.
3) selector switch connects the totalizer of RAM look-up table and digital filtering module simultaneously, is with amplitude information directly or carry out delivering to DAC after the amplitude precorrection through digital filtering module and carry out digital-to-analog conversion by its selection.
4) DAC and signal condition module are responsible for finishing the digital-analogue conversion of the amplitude information after the precorrection, and simulating signal are carried out the amplitude and the skew of filtering, conditioning signal.
5) central processing module mainly is made up of the high performance DSP of a slice (digital signal processor), total system is totally controlled, mainly be responsible for work such as Wave data transmission, phase accumulator control, digital filtering module parameter setting, and selector switch is controlled.
The principle of work of DDS signal source of the present invention as shown in Figure 1, according to the frequency control word that CPU is provided with, phase accumulator produces address signal the RAM look-up table is carried out addressing under the control of sampling clock, and the RAM look-up table is converted to amplitude information with the phase information of waveform, wherein, output signal frequency f
oWith frequency control word f
REG, the sampling clock frequency f
ClkAnd the pass between the figure place N of phase accumulator is:
The amplitude information of RAM look-up table output is switched by selector switch, and selection is amplitude information to be carried out the laggard line number mould of amplitude precorrection through digital filtering module change or directly amplitude information is delivered to DAC and carried out digital-to-analog conversion.
In digital filtering module (referring to Fig. 2), the state control logic unit carries out the change of state under the control of sampling clock, gives data storage cell, tap coefficient look-up table and accumulator element according to the difference output control information of current state.Data storage cell leaves the amplitude information of RAM look-up table output corresponding memory location in and selects corresponding amplitude information to export to multiplier under the control of state logic control module, the tap coefficient look-up table also sends corresponding tap coefficient to multiplier under the control of state logic control module, tap coefficient is by above-mentioned bearing calibration step 1)-4) obtain.Multiplier is finished after the multiplying each other of amplitude information and tap coefficient and the result to be delivered to the accumulator element operation that adds up.The transport function of the digital filtering module that this process realized
Wherein: H (e
Jw) be the frequency response of digital filtering module, ω is an angular frequency, and h (n) is a tap coefficient, and N is a positive integer, and what representative was intercepted counts.Whole amplitude-frequency precorrection digital filtering module all can be in the inner realization of a FPGA (field programmable logic array (FPLA)).
The later Wave data of precorrection send DAC to realize the conversion of digital signal to simulating signal.After handling through the signal condition passage, the signal of DAC output obtains final waveform output.
In the DDS signal source of the present invention, phase accumulator, digital filtering module and selector switch all can be integrated in the inner realization of a high performance FPGA.
The present invention is for providing a kind of amplitude versus frequency characte calibration function of total digitalization based on the signal source (function generator or AWG (Arbitrary Waveform Generator)) of DDS.It is embodied as cheap, and cost performance is very high.The user only need be provided with the correlation parameter of the amplitude-versus-frequency curve of required correction, can obtain the corresponding tap coefficient of digital filter.Revise relevant parameters according to the tap coefficient that obtains, can finish corresponding amplitude versus frequency characte and proofread and correct.And, because the amplitude versus frequency characte that the present invention proofreaied and correct is the output terminal test acquisition by signal source, so the object of calibration is not only because wave filter and DAC sampling keep caused amplitude versus frequency characte unevenness, also comprises parts such as the accurate adjustment of further channel amplitude, decay, amplification because the amplitude versus frequency characte unevenness that influence produced of components and parts value.
Description of drawings
Fig. 1 is the whole theory diagram of DDS signal source of the present invention.
Fig. 2 is the theory diagram of digital filtering module.
Embodiment
Below in conjunction with accompanying drawing, further specify the present invention by embodiment, but the scope that does not limit the present invention in any way.
As shown in Figure 1, in DDS signal source of the present invention, the Wave data synthesis module mainly comprises high-speed phase totalizer, RAM look-up table and sampling clock.The RAM look-up table can be selected according to waveform storage depth and sampling rate that designed signal source will reach, and the general storage depth of selecting is 64K*12bit.The high-speed phase totalizer also needs to design according to the requirement of signal source frequency resolution, satisfies formula Δ f=f
Clk/ 2
N, f wherein
ClkBe the frequency of sampling clock, N is the figure place of phase accumulator, and Δ f is a frequency resolution.The FPGA operating rate is fast at present, and internal resource is abundant, is the first-selection that high-speed phase adds up and designs, and the present invention recommends to use high performance FPGA.
Obtaining mainly of DDS signal source amplitude versus frequency characte function finished by vector network analyzer.Vector network analyzer is actually a Sweep Source and adds a spectrum analyzer, by vector network analyzer, can obtain the amplitude versus frequency characte of the passage of designed signal generator.
Digital filtering module comprises data storage cell, tap coefficient look-up table, high-speed multiplier, high speed totalizer, state control logic unit as shown in Figure 2.According to the signal generator amplitude versus frequency characte of being obtained, can calculate tap coefficient h (0)～h (N-1), its computing method adopt frequency sampling method.Insert in the tap coefficient look-up table of digital filtering module calculating good tap coefficient.Wherein the number of tap coefficient depends on the frequency of operation and the error requirements of system, General N〉16.The enforcement of digital filtering also is completely integrated among the FPGA.
The CPU suggestion is selected for use at a high speed, high performance flush bonding processor, be used for finishing the scheduling of total system, comprise the frequency control word of control phase totalizer, transmit the Wave data of RAM look-up table, the parameter of digital filtering module is set, and selector switch is controlled.
The present invention has obtained application in the DDS of CPCI interface signal source, this signal source sample frequency is 40MSa/S, and maximum output sinusoidal signal frequency is 15MHz.When using the hand-held frequency spectrograph FSH3 of Luo De Schwarz company to record the signal source output amplitude as 500mV, the amplitude table of comparisons before and after proofreading and correct, as shown in table 1.As seen before not proofreading and correct, the uneven maximum disparity of amplitude is 2.5dB, and after overcorrect, gap is reduced into 0.5dB.Well improved the amplitude versus frequency characte of signal source.
Table 1
Claims (7)
1. the synthetic DDS signal generator amplitude-frequency characteristic compensation method of Direct Digital is provided with a digital filtering module and a selector switch between the RAM of DDS signal generator look-up table and digital to analog converter, carries out amplitude versus frequency characte according to the following step and proofreaies and correct:
1) selector switch is set, selects that the data of RAM look-up table output are directly delivered to digital to analog converter and carry out digital-to-analog conversion;
2) obtain the amplitude versus frequency characte function of this signal generator by the output signal of test DDS signal generator;
3) the amplitude versus frequency characte function that is obtained is got inverse;
4) inverse according to the amplitude versus frequency characte function utilizes frequency sampling method to calculate the tap coefficient of digital filtering module;
5) tap coefficient is inserted the tap coefficient look-up table of digital filtering module;
6) selector switch is set, outputs to digital to analog converter after allowing the data of RAM look-up table output by digital filtering module.
2. DDS signal generator amplitude-frequency characteristic compensation method according to claim 1 is characterized in that: in described step 2) utilize vector network analyzer to obtain the amplitude versus frequency characte function of DDS signal generator.
3. a Direct Digital is synthesized the DDS signal generator, comprising:
Carry out the central processing module of overall system control;
Produce the Wave data synthesis module of required frequency wave graphic data;
Wave data is converted to the digital to analog converter of simulating signal from digital signal; And
Simulating signal is carried out the amplitude of filtering, conditioning signal and the signal condition module of skew and output;
It is characterized in that, this DDS signal generator also is provided with digital filtering module and selector switch between Wave data synthesis module and digital to analog converter, digital filtering module carries out the Wave data amplitude information of Wave data synthesis module output to deliver to selector switch after the amplitude precorrection, and selector switch connects Wave data synthesis module and digital filtering module simultaneously, under the control of central processing module, switch, selection be amplitude information that the Wave data synthesis module is produced directly or carry out delivering to digital to analog converter after the amplitude precorrection through digital filtering module and carry out digital-to-analog conversion.
4. DDS signal generator according to claim 3, it is characterized in that: described Wave data synthesis module is mainly by phase accumulator, RAM look-up table, sampling clock and be aided with the logical controlling circuit and form, the frequency control word that phase accumulator is provided with according to central processing module, produce address signal under the control of sampling clock the RAM look-up table is carried out addressing, the RAM look-up table is converted to amplitude information with the phase information of waveform.
5. DDS signal generator according to claim 4, it is characterized in that, described digital filtering module comprises data storage cell, tap coefficient look-up table, multiplier, totalizer and state control logic unit, and wherein: state control logic unit and totalizer are connected with sampling clock in the Wave data synthesis module respectively; Data storage cell is connected with the output terminal of RAM look-up table; The state control logic unit carries out the change of state under the control of sampling clock, data storage cell, tap coefficient look-up table and totalizer are given in difference output control information according to current state, data storage cell leaves the amplitude information that the Wave data synthesis module produces corresponding memory location in and selects corresponding amplitude information to export to multiplier, the tap coefficient look-up table sends corresponding tap coefficient to multiplier, multiplier is finished after the multiplying each other of amplitude information and tap coefficient and the result to be delivered to the totalizer operation that adds up, the amplitude information behind the output calibration.
6. DDS signal generator according to claim 5 is characterized in that, described phase accumulator, digital filtering module and selector switch are integrated in the field programmable logic array (FPLA).
7. according to the described DDS signal generator of arbitrary claim in the claim 3～6, it is characterized in that described central processing module is an Embedded digital signal processor.
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| CN101776935B (en) * | 2009-12-30 | 2012-03-21 | 电子科技大学 | Digital modulation signal generator based on DDS |
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